This invention relates generally to heat-treating furnaces used for brazing and/or annealing of workpieces. More particularly, it relates to an improved method and apparatus for brazing and annealing of a workpiece which includes a furnace having a muffle formed therein and circulation fans disposed in towers which are an integral part of the muffle so as to produce forced convection heat transfer.
As is generally well-known in the art of heat-treating, brazing and annealing require a close tolerance on the temperature control in order to achieve a desired temperature profile or to maintain temperature uniformity within a workpiece. Convection heat transfer has been found to provide faster heating or cooling of the workpiece on a more uniform basis. In such convection heat transfer, the hot gases are typically directed at the workpiece by means of jets or fans, with the fans being the more common means to impart velocity to the gases.
Further, the brazing and annealing processes may also have to be carried out in a special or protective-treatment atmosphere, such as nitrogen or an exothermic gas, which creates conditions that prevent oxidation of the workpiece or imparts other desirable characteristics such as the reduction of oxides. When a combination of rapid heat transfer and a controlled atmosphere has been required, the conventional practice has been to introduce the desired atmosphere into a sealed furnace. For furnaces of the so-called .[.type.]. continuous operation type, these furnaces having an inlet and an outlet are provided with baffles so as to reduce loss of the atmosphere. To protect the atmosphere from contamination due to leakage, these furnaces are heated by gas-fired radiant tube burners or electrically-heated elements such as electrical resistors.
In this manner, such convective heat transfer is attained by means of the fans or jets being mounted in and extending through the side walls or the roof of the furnace. These controlled-atmosphere furnaces generally perform adequately if the furnace insulation does not react with the atmosphere, permit loss of the atmosphere, or cause product/process contamination by deposition of dust resulting from degradation of the insulation. Further, in cases when purity of the atmosphere is critical, the furnace system must be purged for extended periods of time so as to remove the contaminating gases from the voids in the insulation.
In order to prevent the contamination problem as described above, there has been used in conventional practice a gas-impervious metallic inner liner to isolate the workpiece in the controlled-atmosphere form the insulation. Such a prior art convection braze furnace having a lined atmosphere for brazing aluminum heat exchanges is illustrated and described in U.S. Pat. No. 5,147,083 to Gary A. Halstead et al., issued on Sep. 15, 1992. The use of the inner liner suffers from the disadvantage in that it is required to be rigidly positioned with respect to fans, doors, and heating/cooling devices since these items are mounted and sealed to the furnace outer shell.
Another drawback lies in the fact that the need for rigid mounting of the inner liner imposes stresses thereon as a result of differential thermal expansion and contraction between the inner liner and the furnace casing. While various mechanisms have been used heretofore so as to absorb or counteract the stresses due to the differential expansion and contraction, these stresses eventually lead to destruction of the integrity of the liner due to the temperatures normally encountered in the brazing and annealing processes. This problem becomes even more complex because replacement of the liner then becomes necessary which is a very expensive task since the furnace must essentially be completely disassembled for removal of the existing liner and building of a new one in its place.
Moreover, another difficulty results from the necessity of supplying heating or cooling through the radiant tubes which isolates a gaseous media from the furnace atmosphere. This results in low heat transfer rates from these items to the recirculating atmosphere. Also, radiant tubes are temperature limited by the materials of construction that can be used, thereby further reducing heating capacity. Leakage of gases through the radiant tubes and leakage around penetrations are also additional sources of atmosphere contamination.
While there has also been implemented in the prior art muffle-type furnaces in order to overcome the disadvantages of the traditional lined controlled-atmosphere furnaces, these muffle-type furnaces are still not free from all problems. In particular, since the muffle has the capability of being freely movable with the thermal forces there has not been heretofore developed a suitable means for equipping a muffle-type furnace with recirculating fans for imparting convection heat transfer to the workpiece within the muffle so as to produce increased heating rates as well as high thermal efficiencies.
Accordingly, this invention has been conceived and developed to provide a solution to this long-felt need in the industry by mounting circulation fans in towers which are an integral part of the muffle and thus can expand and contract along with the muffle. The present invention represents a significant improvement over the convection braze furnace in the aforementioned '083 patent, which is hereby incorporated by reference.